1. Field of the Invention
The present invention relates to an electron-beam excited plasma generator and, more specifically, to the construction of an orifice through which electrons are pulled out from a discharge chamber of an electron beam generator.
2. Description of the Related Art
Electron-beam excited plasma processing systems are used widely as plasma processing systems including plasma ion plating systems, plasma CVD systems, plasma sputtering systems and plasma etching systems for film deposition, etching and surface modification. An electron-beam excited plasma processing system comprises an electron beam generator which generates an electron beam, and a plasma processing unit having a plasma processing chamber in which a plasma is produced by ionizing a gas by the electron beam to achieve various reactions therein.
The electron beam generator has a cathode, an intermediate electrode, a discharge electrode and an accelerating electrode arranged in that order. When a discharge voltage is applied across the cathode and the discharge electrode, the cathode emits thermions. The thermions convert a gas supplied to the cathode into a plasma. The plasma fills up a discharge chamber between the intermediate electrode and the discharge electrode. When an accelerating voltage is applied across the discharge electrode and the accelerating electrode, electrons are extracted from the plasma and pulled out through an orifice formed in a central part of the discharge electrode, the electrons are accelerated, and an electron beam of a high current is supplied to the plasma processing chamber. The electron beam ionizes or dissociates a process gas supplied into the plama processing chamber into a plasma for processing a wafer.
FIG. 19 is a typical sectional view of a conventional electron-beam excited plasma processing system of a perpendicular beam projection type. A cathode 101, an intermediate electrode 102 and a discharge electrode 103 are disposed coaxially. The discharge electrode 103 is provided in its central part with an orifice 104. An inert gas for producing a plasma, such as argon (Ar) gas, is supplied into a cathode chamber and is ionized into a plasma by a discharge voltage across the cathode 101 and the discharge electrode 103.
A plasma processing vessel 106 has walls made of a conductive material and defines a plasma processing chamber 113. An accelerating voltage is applied to the plasma processing vessel 106 to pull out an electron beam of a high current through the orifice 104 from the plasma produced in a discharge chamber 112. A process gas suitable for a desired reaction to be achieved in the plasma processing chamber 113, such as silane gas or methane gas, is suppplied into the plasma processing chamber 113, the process gas is ionized or dissociated into a plasma by the electron beam. Radicals thus produced are deposited on a workpiece 107, such as a wafer, or ions of the plasma are implanted perpendicularly into the workpiece 107 by the agency of the difference between the potential of the plasma and the surface potential of the workpiece 107.
A pair of solenoids 108 are disposed coaxially so as to surround the orifice 104 of the discharge electrode 103. Currents are supplied in opposite directions to the pair of solenoids 108, respectively. The electron beam 105 traveling through the orifice 104 is constricted by the agency of the inner solenoid 108a to reduce the diameter thereof. A magnetic field created apart from the discharge electrode 103 is cancelled by the agency of the pair of solenoids 108 to spread the plasma throughout the processing chamber 113. The side walls of the plasma processing vessel 106 is protected by a quartz bell jar 109. The bell jar 109 suppresses the deposition of substances on the side walls and can be easily cleared of deposits.
Ar gas supplied into the cathode chamber 111 flows into the plasma processing chamber 113 according to a pressure gradient and is discharged together with the process gas. The workpiece 107 to be processed, is disposed in alignment with the axis of the electron beam 105. A RF bias voltage is applied to a support table 110 holding the workpiece 107 to control sheath ion energy on the surface of the workpiece 107. Water is circulated through the support table 110 to cool the same.
FIGS. 20(a) and 20(b) are graphs representing the condition of the plasma processing chamber 113 in operation. FIG. 20(a) is a graph showing the distribution of surface potential on the workpiece 107, in which distance from the axis of the electron beam 105 is measured on the horizontal axis, surface potential is measured on the vertical axis, curves Vf represent surface potential distributions for different gas pressures in the plasma processing chamber 113 and a curve Vs represents the distribution of plasma potential in the neighborhood of the surface of the workpiece 107. FIG. 20 (b) is a graph showing the distribution of plasma density in the neighborhood of the surface of the workpiece 107, in which distance from the axis of the electron beam 105 is measured on the horizontal axis and plasma density is measured on the vertical axis.
In this conventional electron-beam excited plasma processing system, electrons of the electron beam 105 are accelerated in a direction perpendicular to the surface of the workpiece 107 and the electron beam 105 is projected into the plasma processing chamber 113 and, therefore, high-energy electrons of the electron beam 105 impinge directly on the workpiece 107 if the gas pressure in the plasma processing chamber 113 is low. Consequently, a middle part of the curve Vf indicating the distribution of floating potential sinks deep as shown in FIG. 20(a), and the sheath voltage on the surface of the workpiece 107, i.e., the difference between the plasma potential indicated by the curve Vs and the floating potential indicated by the curve Vf increases. Accordingly, in some types of processes, the surface of the workpiece 107 is damaged by a physical etching action or an intense ion bombardment and the surface of the workpiece 107 cannot satisfactorily processed.
As shown in FIG. 20(a), the floating potential is distributed on the surface of the workpiece 107 in the upward concave curve Vf indicating that the surface potential decreases toward the central part of the surface of the workpiece 107. A potential distribution on the back surface of the workpiece 107 is flat and is the average of the potential of the front surface. Therefore there is a great difference between the potential difference between the surface and the back surface of the workpiece 107 in a central part of the workpiece 107 and that in a peripheral part of the workpiece 107, which, when etching, for example, a gate oxide film for a DRAM, deteriorates or breaks the gate oxide film. Such a trouble can be avoided by increasing the gas pressure in the processing chamber 113 to increase the frequency of collision between the electrons and the molecules of the process gas or by increasing the distance between the orifice 104 and the workpiece 107 to make high-energy electrons fall on the surface of the workpiece 107 after the energy thereof has been reduced. However, if the gas pressure is increased, the energy of electrons decreases sharply with distance from the axis of the electron beam 105 and, consequently, the plasma density increases toward the axis of the electron beam 105 as shown in FIG. 20(b), which affects adversely to the uniformity of plasma processing on the surface of the workpiece 107. Increase in the distance between the orifice 104 and the workpiece 107 is not preferable because the increase in the distance between the orifice 104 and the workpiece 107 entails increase in the size of the system.
FIG. 21 is a diagrammatic view of an electron-beam excited plasma processing system proposed by the applicant of the present patent application in Japanese Patent Application No. 8-68711. This electron-beam excited plasma processing system is of a parallel beam projection type which projects an electron beam and accelerates the electrons of the electron beam in a direction parallel to the surface of a workpiece 201 so that the high-energy electrons may not impinge directly on the surface of the workpiece 201. A cathode 202, an auxiliary electrode 203, a discharge electrode 204 and an accelerating electrodes 205 are disposed coaxially. The electrodes 203, 204 and 205 are provided in their central parts with orifices, respectively.
A gas to be ionized to produce a plasma, such as Ar gas, is supplied into a discharge region 206 and a voltage is applied across the cathode 202, the auxiliary electrode 203 and the discharge electrode 204 by a discharge power supply 208 to ionize the gas, such as Ar gas, and to maintain stabilized discharge. An acceleration power supply 209 applies an accelerating voltage to the acceleration electrode 205 to pull out an electron beam of a high current from the Ar plasma produced in the discharge region 206 into an accelerating region 207. The electron beam travels through the orifice of the accelerating electrode 205 into a plasma processing chamber 210, in which the electron beam dissociates and ionizes a process gas, such as silane gas or methane gas, supplied into the plasma processing chamber 210 to produce a plasma, in the plasma processing chamber 210.
FIGS. 22(a) and 22(b) are graphs representing the condition of the plasma processing chamber 210 in operation. FIG. 22(a) is a graph showing the distribution of surface potential on a workpiece 201, in which distance from the accelerating electrode 205 is measured on the horizontal axis and surface potential is measured on the vertical axis. In FIG. 22(a), a curve B--B represent surface potential distribution in the case of placing the workpiece 201 in a plane B--B in FIG. 21. FIG. 22(b) is a graph showing the distributions of plasma density in the plane B--B (FIG. 21) in the neighborhood of the surface of the workpiece 201 and in a plane A--A (FIG. 21) in the neighborhood of the axis of the electron beam. In FIG. 22(b), distance from the accelerating electrode 205 is measured on the horizontal axis and plasma density is measured on the vertical axis.
The workpiece 201 is supported on a table in the plasma processing chamber 210 with its surface extended in parallel to the axis of the electron beam. Since parts of the electron beam nearer to the accelerating electrode 205 contain more high-energy electrons than parts of the same farther from the accelerating electrode 205, the potentials of parts of the surface of the workpiece 201 nearer to the accelerating electrode 205 are slightly lower than those of parts of the same farther from the accelerating electrode 205. Since the high-energy electrons of the electron beam do not impinge directly on the surface of the workpiece 201, the distribution of floating potnetial on the surface of the workpiece 201 is indicated by a substantially flat curve not having a deeply sunk section, which is different from the curves shown in FIG. 20(a). On the other hand, the density of plasma decreases with distance from the accelerating electrode 205 along the axis of the electron beam as shown in FIG. 22(b). Consequently, when depositing a film on the workpiece, a material for forming the film is deposited at different deposition rates in parts of the surface of the workpiece 201 nearer to the accelerating electrode 205 and parts of the same farther from the accelerating electrode 205, respectively, and a film of an uniform quality cannot be formed. Such a problem is particularly significant when processing a workpiece of a large area.
In either the electron-beam excited plasma processing system shown in FIG. 19 or the electron-beam excited plasma processing system shown in FIG. 21, only electrons which can be ionized in the orifice can be supplied, and there is a limit to the increase of plasma density when electrons are supplied into the plasma processing chamber through a single orifice.
Accordingly, it is an object of the present invention to solve the foregoing problems in the prior art and to provide an electron-beam excited plasma processing system capable of moderating the physical etching action and impacting action of ions, of properly processing workpieces having a large area, and of producing a plasma in a high plasma density and in a uniform plasma density distribution for the efficient processing of workpieces.